A point-to-point radio link is a wireless transceiver system for point-to-point radio communication between two fixed sites. Radio links are commonly used for backhaul in cellular networks, i.e., for connecting network units such as base stations to a core network.
Traditional radio links are often based on microwave transmission, and are as a rule deployed in line-of-sight, LOS, conditions. By LOS condition is meant that a clear line of sight between the two fixed transceiver sites exist, thus the propagation path over which wireless signals propagate is not obscured or blocked by obstacles. The opposite, i.e., when the transmission path is obscured or blocked, and where radio propagation is via diffraction and/or reflection, is referred to as a non-line-of-sight, NLOS, condition.
Radio link transceivers are often used together with highly directive antennas, e.g., disc or horn antennas. Consequently, antenna alignment is an important aspect of radio link deployment, since an erroneous antenna alignment will have an adverse effect on system gain.
An antenna is always associated with an antenna pattern, which in case of a transmit antenna describes the gain of electromagnetic radiation that is emitted in a given direction, and in case of a receive antenna the gain of electromagnetic radiation that is received from a given direction. Reciprocity often holds, meaning that the transmit and receive antenna patterns are often substantially equal in shape. In short, the antenna pattern describes the gain of an antenna as a function of direction (elevation and azimuth).
Herein, both transmit and receive antenna patterns are defined with respect to a global coordinate system, meaning that the antenna pattern changes when the position and/or direction of the physical antenna is changed.
As an example, consider a disc antenna with a single antenna main lobe or beam. When the antenna is deployed, the antenna pattern of this disc antenna essentially describes the direction (elevation and azimuth) and width of the main lobe with respect to a global coordinate system, i.e., not relative to the antenna itself. A change in the direction in which the disc antenna points results in a change of the antenna pattern. An optimization of the antenna pattern of the disc antenna amounts to finding the best direction in which to physically point the main lobe of the disc antenna.
Antenna alignment, i.e., setting the direction (elevation and azimuth) and shape of the antenna transmit and/or receive pattern, is an important aspect of radio link deployment. This is true both when deploying traditional LOS radio links, and also when deploying the more recently developed NLOS type of radio links which do not require a clear line of sight between transceivers. Current alignment procedure is often based on a manual first coarse visual alignment towards a well-known reference point, e.g., a street corner, a roof edge, or the wall of a building, followed by alignment using instrumentation for maximizing a received signal strength or similar.
Once antennas of the radio link have been aligned with respect to each other, i.e., the corresponding antenna transmit and receive patterns have been directed in favorable directions for communication, the alignment is often locked in place and not changed for the remainder of the life-span of the link, unless some rare event occurs which prompts a re-alignment of either or both ends of the radio link. Such rare events may include, e.g., a re-planning of the backhaul network layout, or an exchange of radio link hardware.
In an urban NLOS communication environment there are often multiple possible propagation paths for radio signals between two sites, due to many existing combinations of reflection and diffraction points. This is today used in, e.g., mobile broadband networks to secure connections by utilizing wide beam antennas together with orthogonal frequency division multiplexing, OFDM, and multiple-input multiple-output, MIMO, technologies. However, for high performance point-to-point radio links, antennas having narrow beam antenna patterns are needed in order to maximize energy transfer over the channel, which is needed in order to fulfill the strict requirements on radio performance often posed on this type of communication system.
A problem then, is that it is often difficult to manually choose the best path, or combination of paths, during deployment and initial alignment of an NLOS point-to-point radio link.
During NLOS communication, obstacles in urban environments are utilized for propagation using diffraction and/or reflection. However, the environment may change over time and is out of the control of the radio link operator. As an example, reflection in a building wall may provide different performance when, e.g., windows open or close, or if new objects appear such as signs, sun shades etc. Such events may cause the reflected or diffracted radio signal to change direction and as a consequence the communication system using the NLOS channel may suffer degraded performance.
Thus, another problem is that NLOS radio links cannot be expected to exhibit the same degree of stability in terms of propagation conditions as for the traditional type of LOS microwave links where antenna alignment is often only done once during deployment of the radio link. Existing NLOS radio link installations cannot, and will not, adapt its antenna alignment to changing propagation conditions, and may therefore suffer a performance degradation if the surrounding environment changes.
The problems noted above are especially pronounced for point-to-point radio links, but are not limited to point-to-point radio link communication systems. Hence, similar problems are present in many types of communication systems.